Scientists have made a breakthrough in the development of an Aids vaccine using the genes of HIV, rather than the virus itself, to stimulate immunity.

Scientists have made a breakthrough in the development of an Aids vaccine using the genes of HIV, rather than the virus itself, to stimulate immunity.

An experiment on rhesus monkeys has shown that a so-called DNA vaccine against Aids can prevent infected animals from developing the disease by stimulating the "killer" cells of the body's immune defences. Vaccinated monkeys that were subsequently injected with a highly virulent strain of an Aids virus remained healthy while half of those that were not vaccinated died.

Although such a DNA vaccine is primarily aimed at boosting the chances of someone surviving an HIV attack, the scientists also hope it will have a prophylactic effect by making it less easy for the virus to spread from one person to another.

The findings help to vindicate a radical approach to making an Aids vaccine aimed at cutting HIV transmission rates by lowering the proportion of people in an infected population who carry high concentrations of the virus in their blood.

Norman Letvin, a professor of medicine at Harvard Medical School in Boston, Massachusetts, emphasised the research still has a way to go before it can be said to work in humans.

"We haven't made a vaccine that will prevent Aids virus infections in humans," he said. "However, the findings in this study suggest a vaccine might slow disease progression after an infection has occurred and decrease the likelihood of an infected individual transmitting the virus."

Most vaccines prevent infection by stimulating the production of antibodies but this approach does not work with HIV because of its ability to mutate quickly to many different strains. Aids researchers are therefore concentrating on boosting the killer T-cells of the immune system, which can destroy an HIV-infected cell.

Professor Letvin and his colleagues show in a study published in the journal Science that vaccinated monkeys had five times the number of killer immune cells than unvaccinated monkeys.

Xuefei Shen and Robert Siliciano, two Aids researchers from the Johns Hopkins University School of Medicine in Baltimore, said that even if a DNA vaccine failed to prevent HIV infection it would still be an important weapon in the war against Aids.

"If the results can be generalised to immunisation of humans with HIV-1 vaccines, then we can expect to have vaccines that do not prevent infection with HIV-1 but have a significant effect on the course of the disease, potentially improving the quality of life and the lifespan of infected individuals," they write in an accompanying Science article.

Tomas Henke, a senior scientist at Oxford University's Institute of Molecular Medicine, said the latest findings were encouraging for a parallel vaccine programme in Britain that also used the DNA approach.

"The best vaccines are based on live, attenuated viruses but these are considered too dangerous for HIV. This work shows that using the DNA of HIV can lower the load [amount] of virus in the blood," Dr Henke said. "It's a very significant achievement in that lowering virus load may prevent the onset of Aids and could prevent transmission, which means it would be a prophylactic vaccine."